Low Pitch Slate Roof – Below 20 degrees pitch

Looking to specify natural slate with confidence at low pitches? A series of tests carried out by the BRE have confirmed that with the correct specification, SIGA 65 natural slates perform well at pitches as low as 15°.

Read this article to learn about how we can guarantee a low pitch slate roof and scroll down for advice on how to specify a SIGA Slate low pitch slate roof.

Conventional Low Pitch Slate Roofs

Traditionally, natural slate roofs were not recommended below 20° pitch. However, our technical team always believed that it would be possible to specify natural slates with confidence at lower than 20° if the circumstances were right.

The minimum recommended pitch of a natural slate roof is a function of

the exposure level of the site location;

the local environment in which the roof is constructed;

the size of the slate used;

The head lap, and

The type of fixing method.

Larger slates with longer laps give greater resistance to water ingress, and hook fixings provide better resistance to driving rain than nail fixings. Less extreme local weather conditions also reduce the risk of water ingress.

Conventional low pitch slate roofs are recommended down to a minimum of 20° in areas of sheltered or moderate exposure, providing a 500x300mm slate is used and a headlap of 115mm. Higher exposure or different sized slates necessitated a greater roof pitch of 22.5° or more.

In order to provide you with the opportunity to specify low pitched slate roofs below 20° with confidence, SIGA commissioned the BRE to carry out a set of tests.

One of the Low Pitch Slate Roof test rigs in action at the BRE Wind Tunnel

Low Pitch Slate Roof Testing

The Building Research Establishment (BRE) carried out their Wind Driven Rain Testing of our SIGA 65 slate at low pitches in January this year at their testing centre in Watford.

Two purpose-built rigs were built with nailed and hook fixed 500x300mm SIGA 65 slates at roof pitches of 15°, 17.5° and 20° and with 150mm laps. The rigs were calibrated to simulate a deluge (maximum rainfall with no wind) and high rainfall with high wind speed (wind driven rain) in accordance with prEN 15601:2009.

This procedure includes creating a perspex pressure box underneath the rigs and placing them in front of a wind tunnel and water spray to simulate the precise worst-case wind and rain combinations likely to occur in Northern Europe during any 50-year period.

Test Rig: Illustration from BRE Test Report

During the test the pressure within the perspex pressure box is gradually decreased until the amount of water leaking through the roof exceeds 10g/m2 in five minutes.

The test report shows that the tested SIGA 65 slate roof at 15° with slate hooks performs similarly to the same roof at 20° with nailed slates, and at 17.5° it performs considerably better.

Conclusions of the Low Pitch Slate Roof Testing

Following the successful test results, we are happy to advise specifiers and contractors that they can choose a SIGA Natural Slate roof as low as 15 degrees (sheltered or moderate exposure) and 17.5 degrees (severe exposure) if the slates are hook fixed with a 150mm lap.

Steps to specifying a low pitch slate roof

Set out key steps if you want to use a low pitch slate roof:

Confirm the exposure level of the site using our Annual Driving Rain Index Map. For a 15° pitch the exposure level of site must be sheltered or moderate; if it is severe/very severe a minimum 17.5° pitch is recommended.

If you do have a long rafter length, extreme local topography, or you are working on a tall building, we recommend you consult with us for specific advice.